The invention relates to a new use of a fused material of the metals copper and chrome as contact material for use in vacuum contactor circuit interrupters. More particularly, the invention relates to the contact material itself, the process for its manufacture, the contact pieces made of this material, the process for the contact piece manufacture, and special contact arrangements in vacuum contactors.
Materials made of copper and chrome are known from state-of-the-art engineering. To the extent that they are used as contact materials, they are normally used for vacuum circuit breakers. In such circuit breakers, for example medium-voltage circuit breakers, the key requirement is to be able to switch both high voltages in the kilo-volt range and high currents in the kilo-ampere range. Since the number of switching cycles in circuit breakers is relatively low, their service life can be designed to be on the order of 10,000 switching operations.
Aside from circuit breakers, vacuum contactors are also classified among vacuum switchgear in contrast to vacuum breakers. These are characterized by a very high service life, wherein the desired service life at nominal current is equal to 1,000,000 or more switching cycles.
It is desirable for the materials used for contact components of vacuum contactors, due to this long service life requirement, to meet particularly high standards, especially in terms of their erosion characteristics. During their entire service life, they must be able to reliably handle operating currents of up to approximately 5 kA, and they must also exhibit favorable welding characteristics. A low welding force is desired, so that the opening of the contact pieces is assured even after transient short-circuit currents. It is also desirable to have a material in which the mean value of the chopping current falls significantly below 5 A.
Existing vacuum contactor systems have utilized a compound material based on tungsten and copper as the contact material for vacuum contactors. Therein the tungsten (W) is utilized as the refractory and particularly erosion-resistant component, while the copper (Cu) prevents any overheating of the switching surface, since, it is electrically and thermally a highly conductive material. Since the WCu materials are produced by sintering a matrix of the refractory components and then impregnating the matrix with the low temperature components, the effect of matrix cooling by the material with the lower boiling point can be used against overheating. To reduce the welding force and chopping current, usually additional metallic components, e.g., antimony or tellurium, are added.
The known materials are of tungsten/copper are presently being used successfully for vacuum contactors in the high voltage range, with switching currents up to approximately 3 kA. Due to the increased design requirements regarding switching currents and switching operations, particularly in the low voltage engineering range up to 1 kV, improvements in the contact materials are required. However, tungsten/copper materials can meet these only to a limited extent. The reason lies in the special erosion mechanism of this system. When under arcing loads, due to the extremely high temperatures at the cathode base points of the arc, copper and tungsten are melted and evaporated simultaneously. However, in the perimeter regions of the arc base points, with their lower heating loads the copper tends to be evaporated to a higher degree from the matrix. After the conclusion of numerous nominal current switchings or after a series of high current switching cycles, local copper shortages within the structure may result, thereby generating tungsten-enriched points on the switching surface. A typical structure of a tungsten/copper contact surface which reflects the loading effects imposed by numerous switching cycles therefore includes torn-up and scaly areas.
For dielectric and thermal reasons, these surface structures naturally restrict the switching characteristics and thus the service life of the contact material. Up to now, by the selection of suitable concentration ratios and specific powder particle sizes, the contact material was optimized to the specifically required characteristics. But particularly in the area of low voltage, there is a need to find other contact materials with better erosion/evaporation characteristics.
Investigation of the copper-chrome fused alloys manufactured for vacuum circuit breakers according to the process outlined in U.S. Pat. No. 4,537,745 which is hereby incorporated by reference, has revealed that they are ideally suited as contact materials in vacuum contactors in low and high voltage applications.
An analysis of the metallurgical and particularly thermodynamic conditions of the contacts was made. The desired material had to be a compound material in order to be able to exploit the best characteristics of this contact material classification. It was recognized that the already-described unfavorable erosion/evaporation pattern of tungsten/copper was based primarily on the widely varying vapor pressures of the two metals utilized heretofore. The basis of the invention then was the recognition that a metal combination had to be found whose components, despite otherwise different characteristics should have, as far as possible, similar vapor pressures. Such a combination is specifically provided by a material based on chrome and copper.
As already mentioned, a material based on chrome and copper is already known as a contact material. It was, however, heretofore used advantageously as a contactor piece in high-current, voltage-bearing medium-voltage vacuum circuit breakers. For this operating range the favorable even erosion/evaporation pattern and the resulting dielectric strength were utilized. As in this framework no high numbers of switching operations are required, the substantial erosion/evaporation rate of chrome/copper with high switch-off currents could easily be tolerated.
Particularly due to the suspected erosion/evaporation rate in vacuum circuit breakers, the scientific community was generally considered chrome/copper materials unsuitable for vacuum contactors. The differentiation of the application range of known contact materials is specified in the monograph by A. Keil and others, "Electrical Contacts and Their Materials," Springer Publishing House, 1984, Chapter 4.3, "Switchgear,", particularly table 4.7, page 359.
With this invention, however, it was surprisingly found that a material based on chrome/copper could also be used for vacuum contactors. Thus the prejudice prevailing in the scientific community will be overcome!
Against all expectations, the erosion/evaporation resistance of this material could be demonstrated particularly under contactor conditions, wherein the material easily retained its short-circuit current switchoff capability after experiencing the required switching operations of greater than 10.sup.6 at nominal current. For example, measurements with 600 .ANG. nominal current at approximately 10.sup.6 switching cycles found a change of erosion/evaporation elevation consumption of less than 1 mm per contact piece. At double the nominal current with switching operations of 3.10.sup.5, an erosion/evaporation consumption of less than 1 mm per contact piece was similarly found.
An explanation of the above-specified unexpected consumption behavior is probably due to the different arcing formations in vacuum contactors, in contrast to those of circuit breakers. In particular, the similar vaporization behavior due to largely similar vapor pressure curves of both components represented the main basis for this result.
Tests confirmed that within the framework of this discovery a fused material is suitable for the above-specified use with a composition in components of approximately 25-60%. It was found that the contact material already had satisfactory characteristics after fabrication using fusing processes, preferrably by arc fusion. This material already demonstrated a linear alignment of the Cr dendrites. Thus, so that this linear alignment of the Cr dendrites runs vertically to the switching surface of the contact pieces, the fused material is preferrably reshaped so that its structural alignment is perpendicular to the switching surface. This forming process is best handled by continuous extrusion molding with a greater than 60% reshaping degree.
It was found that the requirements of the material, such as particularly low welding force and low breakdown currents, are generally met by the chrome/copper base material. In special cases, however, the require characteristics can be improved by special additives of tellurium, antimony, bismuth and/or tin. To introduce said additives, various procedures, e.g., smelting, diffusing, or inserting in depressions, can be utilized.
It was also found that vacuum contactors could have an unpaired contact arrangement with contact pieces of fusion materials with different additives, without losing their advantageous characteristics during switching operations.
It is an object of this invention to discover a contact material for use in vacuum contactors which has as good or better switching characteristics as existing tungsten/copper contact materials in their initial condition, and due to better erosion/evaporation characteristics, have a longer and more reliable service life, while retaining the same type of short-circuit current resistance. It is a further object of this invention to discover such a material which also has a low welding force. It is also an object of this invention to discover a method for making the contact material with the above-mentioned switching characteristics. It is also an object of this invention to provide a method for makng the contacts for use in vacuum contactors from the material with the above-mentioned characteristics.